Patent GB-1262206-A (GB)
Title: Speed control of loads driven through electrically controlled clutches
IPC: H02P29/00
Filed: 1969-10-07
Abstract: 1,262,206. Automatic speed control systems. SINGER CO. Oct.7, 1969 [Nov.4, 1968], No.49101/69. Heading G3R. A circuit for controlling the speed of a driven member e.g. a sewing machine through a variable speed coupling has a means for producing a periodic signal of a frequency proportional to the speed of the member, which signal is passed through a "frequency divider" to supply a pulse signal which controls the energization of the driving member in response to a selected number of periods of the signal, the divider including an adjustable element for selecting a variable number of periods, whereby deviation of the driven member from a desired speed alters the energization of the driving member to return of speed to its desired value. The output shaft of a variable speed coupling drives an A. C. tachometer 26 the output from which is rectified and passed unsmoothed to a field effect transistor 30 which produces a sawtooth wave which is converted into a squarewave of fixed height by a circuit including transistor 37. The squarewaves are passed to a "frequency divider" circuit 41 having a variable division range of approximately 30:1 controlled by a potentiometer 42, the slider of which may be manually actuated e.g. by means of a treadle. A capacitor 43 connects the collector electrode of transistor 37 through a diode 44 to a uni-junction transistor 45 and one terminal of another capacitor 46, the other terminal of which is connected to the wiper 47 and a further diode 48. This arrangement works as a "cup and bucket" circuit i.e. periodic charges developed across capacitor 43 are successively passed to capacitor 46 so that a staircase waveform is formed across its terminals the initial amplitude of which is determined by wiper 47. At a determined voltage, uni-junction transistor 45 conducts and resets the circuit hence a determined number of input pulses, dependent on the position of wiper 47 and the ratio of capacitors (C43 + C46)/C43 are necessary for an output to be given via a diode 52 which sets a monostable circuit 53 temporarily so that a transistor 54 is turned on and transistor 55 is turned off this state continuing until the voltage across a capacitor 63 increases to a level where the transistor 55 will be turned on again, the period of charge being approximately 15 milliseconds. A positive pulse developed during the off time of transistor 54 is coupled through a power amplifier stage 64 to a transistor 69 having a normally conductive state plus energizing the coupling winding 10. A "free wheeling" diode 70 shunts the winding. As the frequency division ratio increases the period between pulses from transistor 45 increases hence causing transistor 54 to be off for a longer time and transistor 69 to be on. Similarly when the speed falls, the input frequency to circuit 41 also falls and the pulses passed through diode 52 will also decrease resulting in a longer on time of transistor 69. The coupling winding 10 is fed directly from a full wave rectificer 15 and the control circuit through a smoothing network comprising resistor 20 capacitor 21 the output from which is limited by a Zener diode 22. A capacitor 28 supresses undesirable noise at the input to the arrangement. In an alternative arrangement Fig.2 (not shown) the tachometer may be replaced by a photocell e.g. a photo-transistor (72) to which light is admitted through a slotted disc mounted on the output shaft, and in a further modification the potentiometer 42 is omitted and the capacitor 46 is replaced by a range of capacitors (74-77) selectable by a switch (73).